Medical technology has long been capable of replacing damaged or diseased heart valves through open heart surgery. Such valves have included mechanical devices as well as those using biological material from humans (homograft tissue) and animals (xenograft tissue). The two primary types of prosthetic heart valves known in the art are mechanical valves and bioprosthetic valves. Bioprosthetic valves may be formed from an intact, multi-leaflet porcine (pig) heart valve, or by shaping a plurality of individual flexible leaflets out of bovine pericardial tissue or other materials, and combining the leaflets to form the valve. One advantage of bioprosthetic valves, unlike mechanical valves, is that the patient receiving the valve typically does not require long term treatment with anticoagulants.
The pericardium is a sac around the heart of vertebrate animals which contains lubricating fluid, and bovine (cow) pericardium is commonly used to make individual leaflets for prosthetic heart valves. The bovine pericardium is first harvested from the animal and then chemically fixed to crosslink collagen and elastin molecules in the tissue and increase the tissue durability, before being cut into leaflets.
A good discussion of the various physical properties of fixed bovine pericardium is given in Simionescu, et al., Mapping of Glutaraldehyde-Treated Bovine Pericardium and Tissue Selection For Bio-prosthetic Heart Valves, Journal of Bio-Medical Materials Research, Vol. 27, 697-704, John Wiley & Sons, Inc., 1993. Simionescu, et al., recognized the sometimes striking variations in physical properties of the pericardial tissue, even in the same pericardial sac.
The pericardial sac consists of two distinct elements of tissue. The visceral or serous layer is of very thin translucent tissue most adjacent the heart which is not used to construct artificial heart valve leaflets. This inner layer of the pericardium is conical and surrounds the heart and the roots of the great blood vessels. The parietal pericardial membrane is a thicker membrane of multi-layered connective tissue covered with adipose tissue. The outside fat/adipose tissue is removed (e.g., peeled off) when harvested. The remaining multi-layered fibrous tissue primarily contains collagen fibers with a generally fibrous outer surface and a smooth inner surface. This remaining membrane is used for making the leaflets for artificial heart valves.
A number of steps in a typical commercial process for preparing pericardial tissue for heart valve leaflets are illustrated in FIG. 1. First, a fresh pericardial sac 20 is obtained from a regulation slaughterhouse. The sac 20 is then cut open along predetermined anatomical landmarks, as indicated at 22. The sac is then flattened at 24 and typically cleaned of excess fat and other impurities. After trimming obviously unusable areas, a window 26 of tissue is fixed, typically by immersing in an aldehyde to cross-link the tissue, and then quarantined for a period of about two weeks. Normally, two windows of 4 to 6 inches on a side can be obtained from one bovine pericardial sac. Rough edges of the tissue window 26 are removed and the tissue bio-sorted to result in a tissue section 28. The process of bio-sorting involves visually inspecting the window 26 for unusable areas, and trimming the section 28 therefrom. Subsequently, the section 28 is further cleaned as indicated at 30.
The section 28 is then placed flat on a platform 32 for thickness measurement using a contact indicator 34. The thickness is measured by moving the section 28 randomly around the platform 32 while a spindle 36 of the indicator 34 moves up-and-down at various points. The thickness at each point is displayed at 38 and recorded by the operator. After sorting the measured sections 28 by thickness, as indicated at 40, leaflets 42 are die cut from the sections, with thinner leaflets 42 generally being used for smaller valves, and thicker leaflets being used for larger valves. Of course, this process is relatively time-consuming and the quality of the final leaflets is dependent at several steps on the skill of the technician. Moreover, the number of leaflets obtained from each sac is inconsistent, and subject to some inefficiency from the manual selection process. One solution to this time-consuming manual process is provided in U.S. Pat. No. 6,378,221 to Ekholm, et al., in which a three-axis programmable controller manipulates a pericardial sheet with respect to a thickness measurement head to topographically map the sheet into similar thickness zones for later use. However, even with advanced methods the variability of the bovine pericardium results in an extremely low yield of sheet usable for heart valve leaflets; averaging less than 2 sheets per sac.
Typically, harvested bovine pericardial tissue ranges in thickness from 250 microns up to 700 microns, though most of the material is between 300-700 microns thick.
Valves using flexible leaflets, such as those made of bovine pericardial tissue, have acquired increased significance of late because these valves may be implanted by other than open heart surgery. The valves are constructed using radially expandable stents with flexible (e.g., pericardial) leaflets attached. Implant methods include compressing the valve radially by a significant amount to reduce its diameter or delivery profile, inserting the valve into a delivery tool, such as a catheter or cannula, and advancing the delivery tool to the correct anatomical position in the heart. Once properly positioned, the valve is deployed by radial expansion within the native valve annulus, either through self-expanding stent structure or with an expansion balloon. The collapsed valve in the catheter may be introduced through the vasculature, such as through the femoral artery, or more directly through an intercostal incision in the chest. The procedure can be accomplished without open heart surgery and possibly without stopping the heart during the procedure.
One example of percutaneous heart valve delivery is U.S. Pat. No. 6,908,481 to Cribier and Edwards Lifesciences of Irvine, Calif., which shows a valve prosthesis with an expandable frame on which a collapsible valvular structure is mounted. Another compressible/expandable heart valve is shown in U.S. Patent Publication No. 2010/0036484, also from Edwards Lifesciences. Further examples of such methods and devices are disclosed in U.S. Pat. No. 7,621,948 and US Patent Publication No. 2006/0259136, and the number of other configurations of such valves is exploding as the promise of the technology grows. The disclosures of each of these references are incorporated herein by reference.
These new devices require thinner components that enable crimping of the valve down to a size that can pass through the delivery tool. One limiting component is the thickness of the bioprosthetic tissue. As mentioned, pericardial layers range from 250-700 microns, but only a small percentage of the harvested pericardium falls close to the low end, which is the most useful for compressible/expandable valves.
U.S. Pat. No. 7,141,064 proposes compressing bovine pericardium to reduce its thickness by about 50 percent for use in heart valve leaflets. The compression may also smooth out the tissue surface to reduce thickness non-uniformity.
Despite much research into various bioprosthetic tissue, in particular for heart valve leaflets, there remains a need for thinner and more consistent thickness tissues for use in fabricating smaller delivery profile bioprostheses.